The primary purpose of this study is to compare the relative safety and efficacy of risperidone and placebo in the treatment of children and adolescents with autistic disorder. Risperidone will be more effective than placebo in reducing impulsive aggression, agitation, self-injurious behavior and troublesome repetitive behavior associated with autism; risperidone will result in more sedation (transient) and weight gain than placebo. Assess whether risperidone will be more effective than placebo for improving some aspects of social relatedness in children, adolescents, and young adults with autistic disorder; evaluate whether risperidone has a positive or negative effect on attention, short term memory or motor coordination; evaluate the relationship between steady-state drug levels and clinical response. examine the relationship between CYP450 2D6 and 2C19 genotypes with clinical response, adverse effects, and steady-state drug levels of risperidone; evaluate the relationship between weight gain during risperidone treatment and serum leptin levels.